HeadersDict is a subclass of CaseInsensitiveDict with two new features:
1. Preserve the case of the header key from the first time it was set.
This means that later munging won't modify the key case. (You can
force picking up the new case with `del` followed by setting.)
2. If the value is a list or tuple, unpack it and store the first
element. This is the same as how `Request.add_header()` used to work.
For backward compatibility this commit preserves `Request.add_header()` but
marks it deprecated.
Previously request.headers was a normal dict (albeit with the
request.add_header interface) which meant that some code paths would do
case-sensitive matching, for example remove_post_data_parameters which
tests for 'Content-Type'. This change allows all code paths to get the same
case-insensitive treatment.
Additionally request.headers becomes a property to enforce upgrading it to
a CaseInsensitiveDict even if assigned.
On Python 3, response.msg.keys() contains the same value multiple times if
there are multiple headers with the same value. Work around this by
converting to a set before iterating over it.
There is a weird quirk in HTTP. You can send the same header twice.
For this reason, headers are represented by a dict, with lists as the
values. However, it appears that HTTPlib is completely incapable of
sending the same header twice. This puts me in a weird position: I want
to be able to accurately represent HTTP headers in cassettes, but I
don't want the extra step of always having to do [0] in the general
case, i.e. request.headers['key'][0]
In addition, some servers sometimes send the same header more than once,
and httplib *can* deal with this situation.
Futhermore, I wanted to keep the request and response cassette format as
similar as possible.
For this reason, in cassettes I keep a dict with lists as keys, but once
deserialized into VCR, I keep them as plain, naked dicts.
